This invention relates generally to oilfield downhole tools and more particularly to a rotary steerable drilling apparatus utilizing a universal joint reducing the forces experienced by a bias unit in pushing the bit in the preferred drill path.
To obtain hydrocarbons such as oil and gas, boreholes or wellbores are drilled by rotating a drill bit attached to the bottom of a bottom hole assembly (“BHA”). The drilling assembly is attached to the distal end of a drill string comprised of a plurality of tubulars or a relatively flexible spoolable tubing string commonly referred to as “coiled tubing.” The section comprising the tubing and the drilling assembly is generally referred to as the “drill string.” When a jointed pipe is used as the tubing, the drill bit is rotated by rotating the jointed pipe from the surface or by a mud motor attached to the tubing proximate the drill bit, or preferably both rotation and continuous directional drilling with the BHA. In the case of coiled tubing, the drill bit is rotated by a mud motor. Coiled tubing or flexible tubing may not withstand the rotational torque required in drilling. As either type of drilling occurs, a drilling fluid can be pumped to the drill bit discharging through jets in the drill bit to lubricate and cool the bit and to move rock crushed by the drill bit to the surface. The mud motor uses the hydraulic power of this drilling fluid to power the drill bit.
A substantial portion of current drilling activity involves drilling of directionally deviated wells to fully exploit a given set of geological formations from a single drilling platform. This is especially true of offshore drilling platforms which have daily operating costs. Current drilling programs can provide any number of proposed drill paths to exploit the reservoir from a single location. Such boreholes can provide very complex well profiles. To drill such profiles, bottom hole assemblies are normally provided with a plurality of independently operable force application members to apply force on the wellbore wall during drilling to move the drill bit along a prescribed path.
Continuously rotating directional drilling tools supported by the present invention eliminate slide drilling, improve hole cleaning, increase production rates and reduce the risk of differential sticking. Slide drilling occurs when drilling with a mud motor rotating the bit downhole without rotation of the drillstring from the surface. Slide drilling was required when directional drilling was principally accomplished with bent subs or a bent housing mud motor or some combination of those devices. Slide drilling is eliminated by rotary steerable drilling systems.
Rotary steerable drilling systems are often classified as either “point-the-bit” or “push-the-bit” systems. In point-the-point systems, the rotational axis of the drill bit is deviated from the longitudinal axis of the drill string in the direction sought by the drilling program. In push-the-bit drilling programs, the required directionality is achieved by causing a stabilizer located adjacent the drill bit or remotely from the drill bit to apply an eccentric force on the BHA to move the drill bit in the desired path. Generally, the drill bit is moved into engagement with the borehole face by selective eccentric movement at two other stabilizer locations in the BHA.
As previously noted, rotary steerable drilling apparatus have been developed and are well known in this art for using the flow of drilling fluid to the drill bit to selectively actuate pads or pistons which urge the drill bit along a desired path at the borehole face. These pads may be activated by either hydraulic forces or electromotive forces to move into engagement with the well bore to thereby move or urge the drill bit in a given direction. The force that may be asserted against the pads is generally limited by both the available pressure difference and the piston diameter. Often, the hydraulic force available to push the pad into engagement with the well bore wall is insufficient to both lift the BHA and affixed drill string from the well bore wall and bend the BHA in the desired direction. By strategically integrating a universal joint in the BHA, the effective weight and bending stiffness of the drill string can be significantly reduced and with the same force output, the performance of the rotary steerable drilling apparatus can be dramatically increased.